The invention relates to a method for controlling a multi-clutch transmission of a vehicle, in particularly for controlling whether a gear shift of the multi-clutch transmission should be performed by a power shift or by a power cut shift. The invention can be applied in different vehicles such as e.g. heavy-duty vehicles, trucks, buses and construction equipment.
For a multi clutch transmission such as e.g. a dual-clutch transmission a gear change can be performed either by conventional power cut shift or by power shift. During power cut shift the torque delivered by the internal combustion engine (engine) to the driven wheels is removed before a current, first gear is disengaged. This is done by disengaging the clutch of the current, first gear. This causes a temporarily interruption of torque transferred from an engine to the driven wheels of a vehicle, something which is noticeable for a driver. Subsequently a new, second gear is engaged by engaging the clutch of the second gear. Once again, when subsequently applying the torque this will be noticed by the driver. During power shift the transition from the current, first gear to the second gear is performed essentially without torque interruption. This is accomplished by that the clutch of the first gear gradually slips out while the clutch of the second gear gradually slips in such that the clutches transfer torque during an overlapping period. Thus, power shift essentially without torque interruption is preferred from a driver comfort perspective, but on the downside, power shift may lead to higher wear of the clutches. Also, in order to be able to perform a gear shift by power shift a certain minimum momentary torque has to be supplied by the engine.
If an upcoming gear change of a multi-clutch transmission is performed by power cut shift or power shift may be based on a number of vehicle variables such as e.g. fuel efficiency, vehicle weight, driver requests, road inclination and clutch wear. Power cut shift is generally favourable when the torque delivered by the engine is constant, such as during high way driving at constant speed, where a torque interruption is not that noticeable for the driver. For a gear shift performed when no torque interruption is desired, e.g. when driving uphill, the gear shift is advantageously performed by power shift. If the upcoming gear shift is performed by power shift or power cut shift is also dependent on between which gears the upcoming gear shift is to be performed. Power shift can only be performed for sequential gear shifts whereas during power cut shift intermediate gears can be skipped. Thus, skipping gears, which may be advantageous from a fuel consumption perspective, is only possible if power cut shift is applied.
Since power cut shift involves an interruption in torque delivery, whereas power shift does not, the two different shift methods are experienced differently by a driver. Thus, frequent changes between power shift sequences and power cut shift sequences are undesirable from a driver comfort perspective.
U.S. 2013/0172147 discloses a method and a device for operating a dual-clutch transmission providing a prediction model including at least one simulated shift sequence for the dual clutch transmission. The method includes predicting the time between a first power upshift/downshift and a second power upshift/downshift for the dual-clutch transmission by using the prediction model. If the predicted time between the first power upshift/downshift and the second power upshift/downshift is shorter than a predetermined time at least one parameter for operating the transmission is modified.
The method disclosed in U.S. 2013/0172147 improves the driveability and driver comfort, but there is still a need for further improvements.
It is desirable to provide a method for controlling a multi-clutch transmission of a vehicle, which method improves the driver comfort. It is also desirable to provide a computer program code means, a computer readable medium carrying such a computer program code means and a transmission control system comprising a control unit for running such computer program code means.
Previously known gear shift strategies use a number of predetermined vehicle variables to predict and determine if upcoming gear shift is to be performed as a power shift or as a power cut shift. The vehicle variables may be variables related directly to the vehicle, i.e. vehicle specific variables, variables related to the driving of the vehicle, i.e. driving specific variables, or variables directed to ambient conditions, i.e. ambient variables. Examples of vehicle specific variables are e.g. vehicle load and engine friction. Examples of driving specific variables are e.g. driver requested vehicle speed and vehicle acceleration. Examples of ambient variables are e.g. road inclination and the conditions of the road surface. The predetermined vehicle variables are used to determine current and/or anticipated vehicle behaviour, and based on current and/or predicted vehicle behaviour if an upcoming gear shift should be performed as a power shift or a power cut shift is determined. If power shift is applied a one step up- or downshift is performed essentially without interruption of the torque delivered during the shifting sequence. If power cut shift is applied there will be a momentary interruption of the torque delivered during the shifting sequence but the up- or downshift do not necessarily has to be to the next sequential higher or lower gear. Thus, when power cut shift is applied it is possible to skip gears.
From a driver comfort perspective it is desirable to avoid too frequent gear shifts, and the determination of type of gear shift may e.g. be based on the predicted time before a next forthcoming gear shift is performed. By applying power cut gear shift certain gears may be skipped and thereby the time spent driving on respective gear may be prolonged, whereas if power shift is applied no gears can be skipped. Thus, if the predicted time before a yet forthcoming gear shift is too short it is preferable to use power cut shift and skip at least one gear.
The gear shift strategy may however have the effect that the types of gear shift which is used for consecutive gear shifts changes frequently which may be experienced as annoying by a driver. This problem is addressed by the present method by detecting if a power shift is performed during upshifting in the low gear register, and if so, overruling the previously set gear shift strategy for forthcoming gear shifts and continue to shift gear only by power shift.
It is also desirable that the vehicle behaves in a consistent manner, meaning that the shifting sequence is more or less identical each time e.g. the same uphill climb is driven. If the way the gear shifting is performed is different each time a certain road distance is driven this will also be experienced as annoying by the driver. Such behaviour is also mitigated by applying the method of the present invention.
As previously stated, a known multi-clutch transmission for a vehicle is adapted to be shifted either with a power shift or by a power cut shift. Which type of gear shifting that is performed is controlled by a gear shift strategy, wherein the type of gear shift that is selected by the gear shift strategy is dependent on predetermined vehicle variables. Such a gear shift strategy comprises the method step of;    deciding a forthcoming gear shift as a power shift or a power cut shift dependent on a predicted behaviour of the vehicle based on momentary vehicle variables.
The method of the present invention is characterised in that it further comprises the method steps of:    detecting a power shift to a higher gear in the low range gear register,    wherein if power shift to a higher gear in the low range gear register has been detected the method further comprises the steps of:    overruling the previously set gear shift strategy referred to above and;    controlling the multi-clutch transmission such that a forthcoming gear shift is performed as a power shift.
The detecting operation referred to above comprises registering when the control unit determines that an upcoming shift operation is to be performed as a power shift and alerting the control unit that the previous gear shift was performed as a power shift when a subsequent gear shift is to be performed.
Thus, the known applied gear shift strategy determines if an upcoming gear shift should be performed as a power cut shift or a power shift based on momentary vehicle variables. The momentary vehicle variables, which may be e.g. road inclination, vehicle load and vehicle acceleration, are used to determine a momentary and/or predicted behaviour of the vehicle.
By applying the method of the present invention once a power shift from a lower gear to a higher gear in the low gear register is detected the previously applied gear shift strategy is overruled and forthcoming gear shifts are performed as power shifts independently of what is determined by the previously applied gear shift strategy. This will reduce the number of changes between power shift and power cut shift which will improve the driver comfort.
When driving up a steep uphill slope, or particularly if starting a vehicle in an uphill slope, the gear shift strategy will with considerable certainty determine that an initiating gear shift should be performed as a power shift. By applying the method of the present invention this implies that all forthcoming gear shifts when driving up that uphill slope will also be performed as power shifts. Thus, by applying the method of the present invention the vehicle behaviour will also be consistent each time a steep uphill slope is driven. This is also important for the driver comfort.
The method, including all the method steps, is run by a control unit of the vehicle and is controlled by the software of the control unit. The control may e.g. be an electronic control unit or a transmission control unit. When referring to that the method is performed by a control unit this is not limited to that the method is performed by one single control unit. The method may be performed by a distributed control unit comprising numerous interacting control units and local controllers of the vehicle. This is also referred to as the computer of the vehicle. A computer is by definition a programmable electronic device that can retrieve, store, process and redistribute information, thus the same operations as the control unit, being a single component or a network of interacting components, is provided to perform.
Further aspects of the method comprise the method step of:    evaluating a momentary engine torque against a torque threshold value,    wherein if the momentary engine torque is evaluated to be below the torque threshold value the aspect of the method further comprises the steps of;    overruling any of the previously set, gear shift strategies and;    controlling the multi-clutch transmission such that a forthcoming gear shift is performed as a power cut shift.
As previously disclosed, in order to be able to apply power shift a certain minimum engine torque has to be supplied by the engine. The minimum torque that has to be delivered is set to be the torque threshold value. This minimum engine torque threshold value is not dependent on any vehicle variables but only on that the torque delivered by the engine to the multi-clutch transmission is sufficient in order to perform a power shift. According to this aspect of the method, if the momentary engine torque is below the torque threshold value an upcoming shift operation will always be performed as a power cut shift independently of what is determined by any previously applied gear shift strategies. Thus, previously set gear shift strategies are overruled by this aspect of the method. By applying this aspect of the present invention it is assured that if gear shifting by power shift is allowed the torque supplied by the engine is sufficient to be able to also perform the power shift.
The present invention also relates to a computer program code means for performing the steps of any of, or a combination of, the herein disclosed aspects of the present invention when the computer program is run on a computer. Further, the present invention relates to a computer readable medium carrying a computer program comprising program code means as referred to above. The present invention additionally relates to transmission control system comprising any form of control unit arranged for controlling a multi-clutch transmission, wherein the control unit is programmed to perform the steps of any of or a combination of, the herein disclosed aspects of the present invention.
Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.